PORPOSE: Thatcher illusion is a phenomenon where it is hard to detect the change of local features in an inverted face, although we can easily recognize the change in an upright face (Thompson, 1980). This illusion is thought to be due to the face configural processing. There is no evidence that the squirrel monkey can perceive Thatcher illusion.

METHOD: Subjects were two squirrel monkeys and human participants. The stimuli consisted of the target human face and three kinds of distractor human faces. In the training phase, the discrimination between the target face and two kinds of distractor face (e.g. the face with difference of the eyes from the target face) was trained until the subject obtained the given criteria. In the test phase, the subject was asked to discriminate between the target face and the new distractor face (the face with reversing the eyes from the target face). The faces were tilted at the angles of 45, 135, 225, 315 degrees, there was four conditions as below. A: both the target and the distractor faces were upright, B: the target was upright and the distractor was inverted, C: the target was inverted and the distractor was upright, and D: both the target and the distractor were inverted.

RESULTS AND DISCUSSION: In the training phases, the subject could reach the criteria at or less 5 sessions in all conditions. In the test phases, the condition A, B, and C could immediately reach the criteria. On the other hand, the condition D was needed to 19 sessions for reaching the criteria. These result shows that it is hard to detect the change in an inverted face by the monkey. Squirrel monkey can perceive Thatcher illusion. These results suggest that the squirrel monkey could utilize the configuration of the face like human do.